Solid state image sensors are used, for example, in video cameras, and are presently realized in a number of forms including charge-coupled devices (CCDs) and CMOS image sensors (CISs). CISs sensors are based on a two dimensional array of pixels that are fabricated using CMOS fabrication techniques. Each CIS pixel includes a sensing element (e.g., a photodiode) and access circuitry that are fabricated on a semiconductor substrate, and connected to control circuits by way of metal address and signal lines. These metal lines are supported in insulation material that is deposited over the upper surface of the semiconductor substrate, and positioned along the peripheral edges of the pixels to allow light to pass between the metal lines to the sensing elements through the insulation material. In color image sensors, each pixel also includes a color filter located over the sensing element. An array of microlenses is sometimes located over the metallization layer to focuses light from an optical image through the color filter and the insulation material into the image sensing elements. Each image sensing element is capable of converting a portion of the optical image passed by the color filter into an electronic signal. The electronic signals from all of the image sensing elements are then used to regenerate the optical image on, for example, a video monitor.
The quality of an image generated by a conventional CIS is at least in part determined by the amount of light that reaches the photodiode of each pixel. As indicated above, the photodiode of each pixel covers only a portion of the entire pixel area, with the access circuitry and address/signal lines taking up the remaining CIS surface area. Accordingly, in the absence of microlenses, only a portion of the light incident on the upper surface of the CIS is captured by the photodiodes. Further, when color filters are present, only a portion of the light directed toward a particular photodiode is passed by the color filter, further reducing the amount of captured light that can be used to generate image information. Moreover, because the light must pass through the semi-opaque insulation material of the metallization layer, a portion of the filtered light directed toward each photodiode is reflected or refracted away from the photodiode. Some of this reflected/refracted light may strike an adjacent photodiode, producing blurring and/or inaccurate image color.
What is needed is a CIS that facilitates enhanced image detection by providing a structure for capturing and concentrating substantially all of the light incident on the CIS, and directing the concentrated light onto the CIS's photodiodes.